Stability Constants Data base.

PC-exercise

SCDbase is a database of experimental stability constants for the formation of coordination compounds (also called complexes). The programme has a facility of graphical representations of the relevant equilibria. The aim of this exercise is to introduce the very existence of this data base and some of its facilities.

A The first example deals with glycinate complexes of the metal ions Co²⁺ and Ni^{2+ .}

1. Write down (on a piece of paper) equations for the equilibrium reaction between the metal ions (with the symbol M) and the ligand (with the symbol gly)

Search in the database for stability constants for the above equilibria, will result in a list of many experiments in which the constants are determined with different experimental conditions. In order to work with a not too large number of stability constants (and the relevant experiments), the search is limited by concentrating on the experiments conducted at 25°C (exactly), where the medium is either NaCl or LiCl and the ionic strength is 0.15 M.

2. Search for stability constants for complexes between the metal ion Co²⁺ and glycinate and analogously for Ni²⁺ and glycinate (for which the above experimental conditions are fulfilled). (Notice that the ligand should be entered as the neutral compound - here glycine or C2H5NO2 - or 2-aminoethanoic acid)

3. Store information and the references for the experiments (you may use the notepad facility).

4. Enter the data in the table below.

	log K1	log K2	log K3	log b1	log b2	log b3
Ni^2+
Co^2+

5. Read log K₁ for the Ni²⁺ complex at ionic strength 0.1 M and at the extrapolated ionic strength 0 M.

Now the graphical representation facilities are used. In the first place, the distribution as a function of pH is drawn. – Remember in Main Window to adjust the printer to the landscape format.

6. For the two experiments chosen, click on ”Speciation”.
Under ”Reactants” the concentration of the metal ions are set to 1 mM and the ligand concentration to 100 mM.
Under ”Constants” enter the missing constants, i.e. for cobalt b₃, and for nickel it is log b₃ and the two acidity constants if the ligand (use those given in the cobalt case)
Now, chose ”Calculate as a function of: pH”, then enter the pH interval 2-12. Click on ”Calculate”, and finally on ”Graph”.
The text on the graphs may be edited. Use "edit titles" e.g. with the button with the small letters).
You may add scale lines to the graphs.
Now print the graphs and answer the questions under 7 and 10 using only these graphs.

7. For each metal ion consider: How is the distribution of the different complexes at pH 7.0?

Secondly, the distribution as a function of the ligand concentration is drawn (as pX= -log [X])

8. The window with the graph is closed (click ”Cancel”).
Now chose ”Calculate as a function of: pX”. Set pH to 7.0, and enter the pX interval 0-10. Click ”Calculate”, and then ”Graph”.
Print the graphs (you may add some text on the graph as before) and answer the questions under 9 and 10 using only these graphs.

9. For each metal ion consider: How is the distribution of the different complexes at pX=3.0? To what ligand concentration does pX=3.0 correspond?

10. Using the graphs only you are able to determine which metal ion - Co²⁺ or Ni²⁺ - forms the strongest complexes with glycinate. What is the result?

B The second example deals with oxalato complexes with the metal ions Co²⁺ and Ni²⁺.

1. Write down (on a piece of paper) equations for the equilibrium reaction between the metal ions (with the symbol M) and the ligand (with the symbol ox)

The search in the database for stability constants for these equilibria should be limited by the temperature being exactly 25^°C, the medium NaClO₄ and the ionic strength 1.0 M.

2. Search for stability constants for complexes between the metal ion Co²⁺ and oxalate and analogously for Ni²⁺ and oxalate (for which the above experimental conditions are fulfilled). (Notice that the ligand should be entered as the neutral compound – i.e. oxalic acid)

3. Chose two experiments as basis for the following. Store information and the references for the experiments

4. Enter the data in the table below.

	log K1	log K2	log b1	log b2
Ni^2+
Co^2+

5. For the two experiments chosen, click on ”Speciation”.
Under ”Reactants” the concentration of the metal ions are set to 1 mM and the ligand concentration to 100 mM.
Under ”Constants” enter the missing constants.

6. Distribution of the complekses as a function of pH: Now, chose ”Calculate as a function of: pH”, then enter the pH interval 0-6. Click on ”Calculate”, and finally on ”Graph”.
The text on the graphs may be edited. Use "edit titles" e.g. with the button with the small letters).
You may add scale lines to the graphs.
Now print the graphs and answer the questions under 7 and 10 using only these graphs.

7. For each metal ion consider: How is the distribution of the different complexes at pH 2.0?

8. Distribution as a function of the ligand concentration: Now chose ”Calculate as a function of: pX”. Set pH to 3.0, and enter the pX interval 0-10. Click ”Calculate”, and then ”Graph”.
Print the graphs (you may add some text on the graph as before) and answer the questions under 9 and 10 using only these graphs.

9. For each metal ion consider: How is the distribution of the different complexes at pX=3.0?

10. Using the graphs only you are able to determine which metal ion - Co²⁺ or Ni²⁺ - forms the strongest complexes with oxalate. What is the result?

C The third example deals with ethandiamine as the ligand on the metal ions Co²⁺ and Ni^{2+ .}

1. Write down (on a piece of paper) equations for the equilibrium reaction between the metal ions (with the symbol M) and the ligand (with the symbol en)

The search in the database for stability constants for these equilibria should be limited by the temperature being exactly 25^°C, the medium being KCl and the ionic strength 1.0 M.

2. Search for stability constants for complexes between the metal ion Co²⁺ and ethanediamine and analogously for Ni²⁺ and ethanediamine (for which the above experimental conditions are fulfilled).

3. Chose two experiments as basis for the following. Store information and the references for the experiments

4. Enter the data in the table below.

	log K1	log K2	log K3	log b1	log b2	log b3
Ni^2+
Co^2+

5. Read the log K₁ for the Ni²⁺ complex at 5^°C and at 50^°C.

6. For the two experiments chosen, click on ”Speciation”.
Under ”Reactants” the concentration of the metal ions are set to 1 mM and the ligand concentration to 100 mM.
Under ”Constants” enter the missing constants.

7. Distribution of the complexes as a function of pH: Now, chose ”Calculate as a function of: pH”, then enter the pH interval 2-10. Click on ”Calculate”, and finally on ”Graph”.
The text on the graphs may be edited. Use "edit titles" e.g. with the button with the small letters).
You may add scale lines to the graphs.
Now print the graphs and answer the questions under 7 and 10 using only these graphs.

8. For each metal ion consider: How is the distribution of the different complexes at pH 6.0?

9. Distribution as a function of the ligand concentration: Now chose ”Calculate as a function of: pX”. Set pH to 7.0, and enter the pX interval 2-12. Click ”Calculate”, and then ”Graph”.
Print the graphs (you may add some text on the graph as before) and answer the questions under 10 and 11 using only these graphs

10. For each metal ion consider: How is the distribution of the different complexes at pX=5.0? To what ligand concentration does pX=5.0 correspond?

11. Using the graphs only you are able to determine which metal ion - Co²⁺ or Ni²⁺ - forms the strongest complexes with ethanediamine. What is the result?

D Comparison of the three examples

On the basis of the graphs for the three cases put in order the three ligands, glycinate, oxalate and ethanediamine according to their increasing binding properties relative to Co²⁺ and Ni²⁺.